Conventionally, a study relating to a high electron mobility transistor (HEMT) in which an AlGaN layer and a GaN layer are formed over a substrate by crystal growth, and the GaN layer functions as an electron transit layer has been performed. A band gap of GaN is 3.4 eV, and it is larger than a band gap of GaAs (1.4 eV). Accordingly, a withstand voltage of a GaN based HEMT is high, and it is expectable as a high withstand voltage electronic device for automobiles and so on.
Besides, there are a horizontal structure, in which a source and a drain are disposed in parallel to a surface of a substrate, and a vertical structure, in which a source and a drain are disposed perpendicularly to a surface of a substrate, as structures of the GaN based HEMT.
In the GaN based HEMT, a current flows in a channel because two-dimensional electron gas (2DEG) resulting from a difference between lattice constants exists at the channel positioning in a vicinity of a gate electrode, caused by a structure of the GaN based HEMT even in a case when a voltage is not applied to the gate electrode. Namely, a normally-on operation is performed. On the other hand, it is also conceivable that current flows between the source and the drain when “0” (zero) V is unintentionally applied to the gate electrode, when a power is turned on, when the gate electrode is broken, or the like. Accordingly, a normally-off operation of the GaN based HEMT is desired from a point of view of fail-safe.
It is known that the normally-off operation is enabled by a structure in which a recess is formed at a compound semiconductor layer positioning directly below the gate electrode (gate recess structure) as for the GaN based HEMT having the horizontal structure.
However, in the gate recess structure as stated above, the normally-off operation is enabled, but a threshold voltage is 1 V or less, and a leak current may be generated even though the gate voltage is “0” (zero) V. Besides, it is difficult to use it for an electronic device to which a high voltage is applied if the threshold voltage is 1 V or less, because noise increases and operations become unstable when the high voltage is applied. It is possible to increase the threshold voltage by changing a material from the GaN based material, but it may result in a case that a sufficient current cannot be obtained, or it becomes easy to break only by changing the material.
On the other hand, a study has also been performed to enable the normally-off operation in the GaN based HEMT having the vertical structure, but mass production thereof is difficult by either method.
The following are examples of related art of the present invention: Japanese Laid-open Patent Publication No. 2006-140368; International Publication Pamphlet No. WO 2006/001369; and Japanese Journal of Applied Physics vol. 46, No. 21, 2007, pp. L503-L505.